1560s, from Late Latin hippopotamus, from Greek hippopotamus "riverhorse" (earlier ho hippos ho potamios "the horse of the river"), from hippos "horse" (see equine) + potamos "river, rushing water" (see potamo-). Replaced Middle English ypotame (c.1300), which is from the same source but via Old French. Glossed in Old English as sæhengest.
Ypotamos comen flyngynge. ... Grete bestes and griselich ["Kyng Alisaunder," c.1300]